Take a whiff of mustard or wasabi and you’ll be hit with a familiar burning sensation. That’s the result of chemicals in these pungent foods hitting a protein called TRPA1, a molecular alarm that warns us about irritating substances. The same protein does a similar job in other animals, but rattlesnakes and vipers have put their version of TRPA1 to a more impressive and murderous purpose. They use it to sense the body heat of their prey.

Pit vipers are famed for their ability to detect the infrared radiation given off by warm-blooded prey, and none more so than the western diamondback rattlesnake. Its skills are so accurate that it can detect its prey at distances of up to a metre, and strike at objects just 0.2C warmer than the surrounding temperature. Against such abilities, darkness is no defence.

Like all pit vipers, the rattlesnake’s sixth sense depends on two innocuous pits located between their eyes and their nostrils. With two pits on either side of its head, the snake can even ‘see’ heat in stereo. Each pit is a hollow chamber with a thin membrane stretched across it, which acts as an “infrared antenna”. It is loaded with blood vessels, energy-harvesting mitochondria and dense clusters of nerves. The nerves connect with the visual parts of the snake’s brain, allowing it to match up images of both heat and light. So far, so clear, but until now, no one knew how the membranes actually worked.

Elena Gracheva and Nicolas Ingolia, from the University of California, San Francisco, have solved the mystery but it wasn’t easy. Rattlesnakes don’t give up their secrets readily. Their genes have rarely been sequenced and, in what must be the understatement of the year, Gracheva and Ingolia describe them as “genetically intractable” and “inconvenient subjects for physiological and behavioural studies”. To translate: if you’re looking for a model animal to work with, you’re probably better off with fruit flies and zebrafish than a four-foot serpent with a deadly bite.

The duo suspected that the proteins responsible for the rattlesnake’s heat-seeking powers would probably be found in the unusually large nerve endings that suffuse its pit membrane. They analysed the active genes in these nerve endings, and compared them to those running down the snake’s spine.

In mammals, the two types of nerves have virtually identical portfolios of active genes. The same is true for snakes like the Texas rat snake or the western coachwhip, neither of which can sense infrared. But in the rattler, Gracheva and Ingolia spotted an unmissable difference – a single gene that encodes the TRPA1 protein was 400 times more active in the pit nerves than the spinal ones.

In humans, TRPA1 is activated by allyl isothiocyanate, the chemical that gives wasabi and mustard their kick. The rattler’s protein, which is 63% identical to ours, responds to the same chemical but more weakly. It is, however, exquisitely sensitive to heat. At room temperature, the protein is idle. But anything over 27.6C will set it off and the higher the temperature, the more active the protein. By comparison, a rat snake’s version of TRPA1 is also sensitive to heat, but it responds more weakly than that of the rattler, and at a higher threshold temperature.

Two other groups of snakes, the pythons and boas, can detect infrared radiation, although their technology is 5-10 times less sensitive than the sophisticated viper hardware. They also have pits but theirs are spread across their snouts, are simpler in structure and have fewer nerve connections. But Gracheva and Ingolia found that they have independently co-opted the same molecule in their pursuit of hot sensory action, even though their ancestors diverged from those of vipers 30 million years ago.

In invertebrates like flies and worms, TRPA1 also plays a role in sensing temperature changes. In vertebrates, it’s more to do with sensing foul chemicals and possibly cold temperatures but it seems that three groups of snakes have revived the ancient function of these proteins.

And now a question for the audience: Infrared detection seems like an extremely valuable skill, so why is it that only two groups of snakes have evolved it? If it’s all done by co-opting the same apparently malleable protein, why isn’t the strategy more common? I asked the lead author but he had no answer either. Any thoughts?

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Maybe the reason not more snakes go down the same path as vipers and rattlers for the extra sensory ¨device¨ is because they might not need it.
Maybe the snakes the two kinds of snakes that have it evolved to having that refined because it was a necessity more than a luxury!
thanks,
.1.

Probably not – their receptors aren’t as sensitive to mustard oil as ours are, and besides, they have their regular eyes too 😉 Also, I’m not about to rub mustard on a rattlesnake’s face in order to find out…

Not sure how this might compare with other snakes, but possibly the infrared sense is useful because their prey are crepuscular, nocturnal or living in burrows where it is more difficult to hunt by scent, vision or hearing?

@Kargoneth, I was thinking the same thing. It’s like looking at the evolutionary development of the eye again, except for IR frequencies instead of visible light.
This is hard to evolve. It probably required developing an evolutionary advantage that allowed the protein to drop off in chemical sensing, and a good reason for a heat-sensing exaptation to take hold. The development of venom, for example, could provide enough advantage to allow plenty of genes to shuffle around.
Not that that says anything about the boas and pythons. There, being ambush predators probably encourages thermoreception. It would be very useful in the detection of animals running by on a tree limb at night, or near a water hole. Stake out an active location, and the heat receptors will allow you to ambush rapidly-moving prey (which, importantly, will not be around long enough to localize by scent). Confirming that something you see moving a foot or two away is edible is important. Apparently, again relying on wikipedia, it’s evolved multiple times in pythons and boas, which makes sense.
Incidentally, the effective difference in useful range at night for rattlesnakes and boas or pythons is likely to be larger than the 5-10x difference in the sensors themselves. With equivalent sensors, desert animals would be able to detect prey at about a 70% greater distance due to lower nighttime background temperature. (Assuming 36C body temperature, and 30C average background for rainforest vs. 18C average background for desert.) This increases the usefulness of a really good sensor for desert-dwelling vipers like the Western diamondback, once the basic thermoreception is developed.

Heat sensors are surprisingly useless in robotics, and I suspect the reason are the same long-wave IR detection is rare in animals.
First, you have your own body heat. Unlike your eyes, your IR sensors need to deal with being embedded in a body that constantly shines in the same wavelengths you’re trying to detect.
Second, long IR has very low spatial resolution, low energy differential, is quickly dispersed in air (and gets absolutely nowhere in water), and “contaminates” everything around it. Imagine how useful vision would be if everything faintly glows; if the difference between the strongest and weakest light is that between a 20 watt and 40 watt bulb (rather than, say, sunlight and deep shadow); if you’re constantly in a dense fog that stops all vision further than 4-5 meters; and if strong light sources make things -a and the air itself – around them glow brighter after a while.
It’s a pretty sucky detection medium, in other words, and it really only makes sense if you have absolutely nothing better available. Reptiles are close to ambient temperature, making long IR at least feasible to use, and they have lost the sense of hearing that most other night hunters seem to use instead.

@Janne, on the other hand, it’s more difficult for mammals to camouflage your IR emissions than their visual spectrum reflections, so there are some advantages to seeing in that band.
The ground squirrel defense is the only known confuser I can remember hearing about.

Given the generally sorry state of snake phylogeny, how sure can we be that the two groups of snakes developed this ability independently? Last I read, snake phylogeny is very poorly understood, so how possible is that that these snakes may be more closely related than is thought?

It would be fascinating to determine how far back in the rattlesnake lineage the thermosensing pits extend. When did they develop and does that correspond to the presence of warm/hot prey? Were there ancient lizards with this ability? Do the pits correspond to structures in the skull so they could be traced in the fossil record?

Thanks for posting this- will link to our FB. Too bad the photo is of an eastern diamondback, C. adamanteus, not a western diamondback, C. atrox.
My thoughts on others not getting this– many Elapids, such as cobras, hunt in wet conditions, where temp. differences might not be as easy to ‘see’. Diurnal colubrids (such as the coachwhip) are so fast they may not need the heat sensory stuff. But for many other snakes it sure does seem useful, so I don’t have a broad explanation other than quirky evolution.
Also, @Jdaniel, I don’t think phylogeny is so poorly understood that we think pythons are close relatives of pitvipers.

“Heat” is molecular kinetic energy and cannot be directly sensed at a distance. You can sense things that feel warm without directly touching them because all objects emit IR radiation at wavelengths proportional to their surface temperature.

Apologies for the spam, but after reading the paper it seems to support a mechanism of ‘thermotransduction rather than phototransduction’, which means the distinction between IR and heat is indeed significant.

I think the main reason vipers, boas, and pythons have heat sensitive pit organs may be because of their similar hunting methods. Most vipers, boas and pythons are ambush predators. They sit in one spot and wait for prey to come to them. More than likely, their ancestors were ambush predators as well and heat sense is an obvious advantage; which is why three groups of snakes share this feature. Most other snakes are active foragers which mostly rely on chemical cues to find their prey, and heat sense may help, but probably only slightly when compared to its use for an ambush predator.